Sea Shells of Southern Africa

Introduction:

In 1848 Dr. Ferdinand Krauss published "Südafrikanische Mollusken", the first book devoted solely to South African sea shells. Since then numerous articles have appeared in journals and many books have been published. However, these books, with a few exceptions, are now out of print. The purpose of this guide is to provide the reader with a concise reference work to the many sea shells found along the southern African shores, by presenting detailed descriptions and other useful information on the shells most often encountered.

A few uncommon and rare shells are also illustrated. WHAT IS A SHELL? Shells that wash up on the shore once belonged to small animals known as molluscs. A mollusc is a soft-bodied animal that is unsegmented, although the body can be divided into four basic parts: foot, head, mantle, and the viscera or internal organs. The foot is a muscular structure that enables the mollusc to cling, crawl, burrow in sand, or even swim. The mantle secretes the shell, which functions as an external skeleton that supports and protects the soft body with its organs.

A shell is composed mainly of calcium carbonate and a protein matrix of conchiolin, secreted by cells that line the edge of the mantle. Calcium is deposited in layers to thicken the shell during growth. A form of calcium known as aragonite reflects light, pro-ducing the shiny, mother-of-pearl lustre on the inner surface of, for example, abalones and pearl oysters.

The sculpture, or decoration, of a shell results from folds and fringes on the mantle, forming a variety of unique scales, ridges, spines and tubercles. The colour and patterns are created by the intermittent secretion of pigments from metabolic waste derived from the diet of the animal. These pigments are mixed in with the calcium while it is still soft.

Not all molluscs have an outer skeleton in the form of a shell: the cephalopods (octopuses, squid and cuttlefish) have an internal structure, the cuttlebone, which is used for buoyancy regulation. The bottom-dwelling nudibranchs (sea slugs) have no external skeletal structure for protection - they defend themselves by secreting toxic chemicals. Their vivid colour patterns, which can be most attractive, warn predators of their unpleasant taste and toxicity. [...]